In the last decade, a great deal of work has been done by the scientific community on alkali metal secondary battery systems. The most worked upon system has been the sodium sulfur secondary battery. In this case, sodium is used as a liquid anode and sulfur is used as a liquid cathode. A solid electrolyte is provided which is conductive to ions of the liquid anode. Suitable electric terminals are used to put power into the battery to recharge the same or to withdraw power during discharging thereof.
In a normal discharge operation, individual sodium atoms give up an electron to the external circuit, pass through the electrolyte as a sodium ion, and react with the sulfur of the cathode which has received the electron from the external circuit to form sodium sulfur reaction products. During recharge, the sodium sulfur reaction products give up electrons to the external circuit, thus producing sodium ions. These sodium ions pass through the electrolyte and are recombined with the electrons in the anode compartment to once again form sodium.
During discharge of the sodium sulfur cell, it is desirable to have at least enough sodium available to convert all of the sulfur in the cathode compartment to Na2S3. If enough sodium is not available, the discharge will terminate early (when all sodium is exhausted) before the average melt composition in cathode has reached Na2S3. This results in decrease of the energy of the cell or reduced energy density. However, in the sodium sulfur system one does not desire to keep a large amount of the anode material available in the vicinity of the sulfur in the cathode compartment because if the compartment wall is ruptured, these materials may intermix and cause a heating of the battery that may damage the material holding the battery together.
It is therefore a principal object of this invention to provide a volume efficient sodium sulfur battery system in which a substantial amount of the sodium anode material is available during discharge of the battery, but the sodium is kept available for the reaction in such a manner that there is not a large volume of the sodium reactant located adjacent the compartments in which the sulfur reactant is stored.
In the recharging of a sodium sulfur battery system, generally the cathode compartment will contain sulfur and decreasing amounts of sodium polysulfide materials. The sulfur, which is lighter than the polysulfide materials, is a material which is not electrically conductive. During recharging, the sulfur forms at the electrolyte graphite felt interface in the cathode compartment. If this electrically insulating sulfur film is not removed and replaced with conductive sodium polysulfide material, the recharging process of the battery is effectively terminated.
It is therefore another principal object of this invention to provide a volume efficient sodium sulfur battery in which polysulfide materials in the cathode compartment are constantly moved into contact with at least part of the electrolyte associated therewith during the recharging of the battery by a gravity feed mechanism. This insures that so long as sodium polysulfide materials are contained in the cathode compartment, they will be available adjacent the electrolyte for the purpose of carrying out the recharging operation. In the recharging operation, the sodium polysulfide material gives up electrons to the external circuit to produce sodium ions which migrate through the electrolyte to be reassociated with the electrons in the anode compartment to once again form sodium.
U.S. Pat. No. 3,932,195 issued Jan. 13, 1976 for "Electric Cells", discloses a sodium sulfur battery cell in which an extended surface area current collector is sandwiched between facing portions of a solid electrolyte also of extended surface area. The solid electrolyte provides a container for liquid sulfur. Sulfur is generally located in contact with most of the area of the electrolyte for the purpose of carrying out the reversible electrochemical reaction with the sulfur contained in the cathode compartment. Several figures of the drawing suggest construction of the cell in a manner in which a plurality of rectangular cathode sulfur containing compartments are provided in various configurations.
U.S. Pat. No. 3,811,943 issued May 21, 1974 for a "Mass Transportation Electrode for Energy Conversion Device", shows a structure useful in a sodium sulfur battery. In this construction, a container 10 is used to contain sodium which is conducted by means of plurality of electrolyte tubes 20--20 into reactive association with sulfur contained in a second container indicated by the numeral 14.